Crude oil washing (COW) is a crucial procedure employed in the maritime shipping industry to enhance the efficiency of cargo transport and minimize environmental impact. This technique involves the use of crude oil itself to clean cargo tanks, significantly reducing the residue that can remain in the tanks after the discharge of oil cargo. By washing the tanks with the same oil being transported, vessels can maximize their cargo outturn while ensuring compliance with international regulations aimed at reducing marine pollution.
During COW, crude oil is pumped from the ship's slop tanks, where it is preheated, and then sprayed at high pressure onto the walls of the cargo tanks. This process effectively dislodges the residual oil that tends to cling to the tank surfaces due to the sticky nature of crude oil. As a result, the amount of cargo 'remaining on board' (ROB) is significantly decreased, which is particularly beneficial in an industry where the price of oil is consistently high
To maintain operational efficiency, a good trim of up to 5 meters must be maintained during COW. The washing systems used are similar to those employed for water washing, consisting of fixed machinery with pumping systems designed for delivering and stripping tanks. According to MARPOL Annex I regulations, approximately 25% of the tanks should be washed on a rotational basis to control residues, and tanks are generally not required to be washed more than once every four months
The advantages of COW are manifold:
Reduced Operational Sea Pollution: By using crude oil for washing, the discharge of oil-contaminated water into the sea is minimized.
Improved Cargo Outturn: COW allows for a greater quantity of crude oil to be transported, as less residue remains in the tanks.
Reduced Tank Corrosion: Regular cleaning helps to prevent corrosion and damage to the tank structure.
Less De-sludging Time: When preparing for dry docking, less time is needed to de-sludge tanks that have undergone COW.
Less Salt Water Discharged to Refineries: This reduces the potential for contamination in the refining process
While the benefits of crude oil washing are significant, there are also some disadvantages to consider:
Increased Workload: The process can add to the workload during discharge operations.
Extended Discharge Time: COW may extend the overall discharge time by 6 to 12 hours, affecting scheduling
Higher Risk of Pollution in Port: There is an increased risk of spills during the washing process, necessitating careful monitoring and compliance with environmental regulations
To mitigate these risks, several precautions must be adhered to during COW operations:
A port-specific plan must be established to address potential hazards.
The COW line must be pressure tested before use to ensure integrity.
Effective communication among crew members should be established.
A responsible person should check for leaks both before and during the washing process.
The vessel must be inerted, ensuring that the oxygen content is below 8%.
The washing machines should be functioning correctly, and the washing program should be properly set
